Vacuum refining method for molten steel

ABSTRACT

The present invention relates to a molten steel refining method for refining molten steel, the carbon content of which is not more than 0.1 weight %, by blowing oxygen gas for decarburization at a blowing speed so that a cavity, the depth of which is 150 to 400 mm, can be formed on the surface of molten steel in a straight barrel type vacuum refining apparatus in which a straight barrel type vacuum vessel having no vessel bottom and a ladle are arranged. When necessary, the above decarburizing processing conducted by blowing oxygen gas is combined with: an Al heating process in which Al added into the vacuum vessel is burned by oxygen gas blown into the vacuum vessel at a blowing speed such that the cavity depth can be 50 to 400 mm; a degassing treatment conducted in a high vacuum condition; a desulfurizing treatment in which a desulfurizing agent is blown into the vacuum vessel; or a burner heating treatment in which a combustion improving agent is blown together with oxygen gas, wherein each treatment except for the high vacuum degassing treatment is conducted in a degree of vacuum of 100 to 400 Torr.

BACKGROUND OF THE INTENTION

1. Field of the Invention

The present invention relates to a vacuum refining method for moltensteel. More particularly, the present invention relates to a vacuumrefining method for refining molten steel with a straight barrel typevacuum vessel having no vessel bottom.

2. Description of the Related Art

In a vacuum refining furnace, oxygen gas is blown onto molten steel tobe refined by means of top-blowing. The objects of blowing oxygen gas bymeans of top-blowing are described as follows. The first object is"decarburization" in which oxygen gas is reacted with carbon containedin the molten steel when oxygen gas is blown. The second object is "Alheating" in which the temperature of molten steel is raised when Aladded to molten steel is burned by oxygen gas blown onto the moltensteel by means of top-blowing. The third object is "desulfurization" inwhich flux, such as lime, is added to molten steel together with carriergas. The fourth object is "burner heating" in which oxygen gas andcombustion improving gas of a hydrocarbon, such as LNG, are blown bymeans of top-blowing so as to heat a vacuum vessel and suppress theadhering metal.

Conventionally, DH is known as a vacuum refining furnace composed of astraight barrel type vacuum vessel and a dipping snorkel. However, inthe case of DH, a vacuum vessel to circulate molten steel goes up anddown, and no molten steel exists in the vacuum vessel when it is movedto the uppermost position. Accordingly, in the case of blowing oxygengas by means of top-blow, oxygen gas directly collides with the bottomof the vacuum vessel. Therefore, refractory material of the vesselbottom is severely damaged by the colliding oxygen gas. For the abovereason, a method of blowing oxygen gas from a top-blowing lance has notbeen adopted at all.

Although it is not a case of vacuum refining, a secondary refiningfurnace in which the top-blowing of oxygen gas is conducted with astraight barrel type dipping snorkel is described as "CAS-OB Method" inS1086 of vol. 71 of "Iron and Steel" published in 1985. The object ofthe above method is to raise the temperature of molten steel by burningAl. However, the following problems may be encountered according to theabove method. In the above method, it is impossible to conduct pressurereduction processing. Accordingly, when it is necessary to conduct avery low carbon steel melting processing and a dehydrogenationprocessing together with "Al heating", it is necessary to provideanother refining furnace, so that the equipment cost is increased. Sincethe operation is conducted under atmospheric pressure, molten steel cannot be sufficiently agitated, and the heat transfer efficiency is low.In order to improve the heat transfer efficiency, it is necessary toextend the processing time.

In the decarburizing reaction treatment conducted for producing ultralow carbon steel by means of top blown oxygen in a region, the carbonconcentration is not more than 0.1%. Since the carbon concentration isvery low, oxygen gas which has been blown out by means of top-blowingtemporarily generates an iron oxide on the surface of molten steel, andthis iron oxide reacts with and is reduced by carbon contained in themolten steel. In order to facilitate the reducing reaction, it isnecessary to raise the hot point so as to form an advantageous conditionfrom the viewpoints of thermodynamics and reaction speed. Therefore, itis necessary to conduct a so called hard-blowing operation in which thetop-blown oxygen is made to collide with the surface of molten steel athigh jet intensity.

With regard to a molten steel refining method in which an RH type vacuumrefining apparatus having a vessel bottom is used and a water-cooledtype top-blowing lance inserted into a vacuum vessel from an upperportion blows out a jet stream of oxygen into the vacuum vessel forrefining molten steel, an example is shown in Japanese Unexamined PatentPublication No. 2-54714. Therefore, this molten steel refining method iswell known.

FIG. 8 is a schematic illustration showing a refining method of moltensteel conducted by a conventional RH type vacuum degasifying apparatus.The operation will be explained below. There is provided a snorkel ofup-leg 23 at the vessel bottom 22 of the vacuum vessel 21. Gas is blowninto the vacuum vessel 21 from a lower end of the snorkel of up-leg 23,so that the molten steel 24 can be sucked up from a ladle 25 to thevacuum vessel 21. In the vacuum vessel 21, an oxygen jet 27 is blown outfrom a top-blowing lance 26 to the surface of the molten steel 24. Inthis way, the molten steel 24 is subjected to decarburizing processingand Al heating, and the thus processed molten steel 24 is returned tothe ladle 25 via a snorkel of down-leg 28. When the molten steel 24 iscirculated between the ladle 25 and the vacuum vessel 21 in this way, itis continuously processed.

However, when oxygen is fed from the top-blowing lance 26 in the RH typevacuum refining apparatus described above, since the vacuum vessel 21has a vessel bottom 22, the operation is restricted in various ways, andthe following problems may be encountered.

In the RH type vacuum refining apparatus, vacuum necessary for suckingup the molten steel 24 from the ladle 25 so as to make the molten steel24 reach the vessel bottom 22 of the vacuum vessel 21 is usually notmore than 200 Torr. In order to circulate the molten steel 24 afterthat, vacuum is further enhanced, and it becomes necessary to maintain ahigh vacuum of not more than 150 Torr. Further, when oxygen gas is blownout from the top-blowing lance 26 in a reduced pressure condition, it isnecessary to maintain a high vacuum condition. Unless a high vacuumcondition is maintained, an oxygen jet 27 collides with the vesselbottom 22, and the refractory material at the vessel bottom is damagedbecause the molten steel depth T is small. Accordingly, in the case ofconducting the hard blowing operation, the following restrictions mustbe observed. In order to keep the depth L of a cavity 29, for example,it is necessary to keep a very high vacuum of about 10 Torr so that thehead of molten steel can be raised to maintain the depth T of moltensteel on the vessel bottom 22 in the vacuum vessel 21.

In the case where oxygen is blown out from the top-blowing lance at alow degree of vacuum, a quantity of molten steel to be sucked is small,so that the depth T of molten steel in the vacuum vessel 21 is small.Therefore, for the same reason as that described above, the oxygen jet27 collides with the vessel bottom 22, and the refractory material atthe vessel bottom is damaged. Therefore, the depth L of the cavityformed by the oxygen jet 27 is restricted. As a result, it is impossibleto conduct the hard-blowing operation, and it is necessary to conduct aso called soft-blowing operation in which the top-blown oxygen is madeto collide with the surface of molten steel at low jet intensity.

Consequently, in the RH type vacuum refining apparatus, the followingproblems may be encountered. When oxygen gas is blown out in a reducedpressure, it is restricted as described above. Since it is impossible toconduct a hard-blowing operation in a low degree of vacuum at thebeginning of the treatment, the reduction of iron oxide is delayed andthe decarburizing reaction speed is lowered. In addition to that, thejet speed of the oxygen gas is low. Therefore, after the lance has beendischarged, oxygen in the periphery of the jet reacts with CO gas in theatmosphere, so that CO₂ is generated. That is, the post combustion isactively conducted, for example, at a rate of post combustion that isnot less than 20%. Accordingly, the temperature in the vessel isunnecessarily raised and the refractory material of the vacuum vessel isdamaged.

On the other hand, when a vacuum refining apparatus, which will bereferred to as a straight barrel type vacuum refining apparatushereinafter, is used for refining, in which a lower portion of thestraight barrel type vacuum vessel having no bottom is dipped in themolten steel in the ladle, it is possible to blow out oxygen even in alow degree of vacuum because there is provided no vessel bottom. Whenoxygen is blown out by means of top-blowing in the above refiningapparatus, it is necessary to maintain the vacuum refining apparatus ina low degree of vacuum in order to facilitate the decarburizingreaction. The reason is that it is difficult for iron oxide to flow outfrom the vacuum vessel in the case of an unnecessarily high degree ofvacuum, so that the decarburizing efficiency is lowered. To thecontrary, when the degree of vacuum is too low, the circulation ofmolten steel is deteriorated, and molten steel can not be sufficientlymixed. Accordingly, the decarburizing efficiency is lowered.

Examples in which stainless steel is refined by means of top-blowing inthe above straight barrel type vacuum refining apparatus are disclosedin Japanese Unexamined Patent Publication No. 1-156416, No. 61-37912,No. 5-105936 and No. 6-228629. In the above examples, the carbonconcentration at which decarburization starts is in a high carbonconcentration range of not less than 0.2%. Further, in the above patentpublications, there is no specific description about the oxygen blowingcondition.

In the decarburizing reaction conducted at the aforementioned highcarbon concentration, the top-blown oxygen directly reacts with carbonin the molten steel since the carbon concentration is high. In the abovecircumstances, no iron oxide is generated. Accordingly, even ifconverter slag exists in the vacuum refining apparatus, no problems arecaused. Also, since the carbon concentration is sufficiently high, theagitating and mixing characteristic and the decarburizing efficiency arenot affected. Accordingly, in this case, the higher the vacuum in thevacuum refining apparatus is, the more effectively the decarburizationcan be conducted. In the above well-known documents, Japanese UnexaminedPatent Publication No. 5-105936 discloses an example in which the degreeof vacuum is maintained at 200 Torr, and Japanese Unexamined PatentPublications No. 1-156416, No. 61-037912 and No. 6-228629 discloseexamples in which the degree of vacuum is kept at 100 Torr or 50 Torr.

In the case where the carbon concentration is high, from the viewpointof the principle of decarburization, the higher the degree of vacuum is,the more advantageous the effect that can be provided. However, in orderto keep the vacuum refining apparatus in a high vacuum condition, theinvestment in plant and equipment is necessarily increased for thevacuum pump system because a large quantity of CO gas is produced, andfurther molten steel splashes violently in the process. Therefore, it isnecessary to increase the height of the apparatus for the prevention ofsplash. As a result, the investment in plant and equipment is increased.For the above reasons, in the above examples, the degree of vacuum ismaintained at 100 Torr or 50 Torr. In the above well known documents, itis described that refining is continued until the carbon concentrationbecomes 0.01 to 0.02%. However, metallurgical effects are not shown whenthe carbon concentration is restricted to a value lower than 0.1%.

However, as described later, in a high vacuum condition in which thedegree of vacuum is higher than 105 Torr, it is difficult for slagparticles in the molten steel to flow out from the vessel, so that thedecarburizing oxygen efficiency is low. Therefore, in the case of adegree of vacuum lower than 195 Torr, the agitating energy is reduced,and the molten steel can not be agitated and mixed sufficiently. Forthis reason, the decarburizing efficiency is lowered.

Japanese Unexamined Patent Publication No. 7-179930 discloses an examplein which plain carbon steel was refined under the condition that thedegree of vacuum was maintained at 200 Torr and oxygen was blown bymeans of top-blowing so that the carbon concentration was in a rangefrom 0.03% to 0.001%. In this case, the post combustion rate was notless than 78%, and the decarburizing oxygen efficiency was very low. Thereason was that the cavity depth, which was found by calculation usingthe expression described later, was only 52 mm. That is, the oxygen gascollided with the molten steel in the manner of soft blowing. Also, itcan be considered that the degree of vacuum was too low, so that themolten steel was not agitated and mixed sufficiently and thedecarburizing efficiency was further deteriorated. Japanese UnexaminedPatent Publication No. 6-116627 discloses a method in which the moltensteel, the carbon concentration of which is 0.03 to 1.0%, is subjectedto a top-blown oxygen, and the vacuum P is controlled in accordance withthe equation of P (Torr)=a+980× %C! (a=170 to 370). The object of thismethod is nitrogen removal. Although there is no description about thedecarburizing efficiency, the degree of vacuum is 199 to 399 Torr whenthe carbon concentration is 0.03% which is the lowest value. In the lowdegree of vacuum described above, the stirring energy is lowered.Therefore, the molten steel can not be stirring and mixed sufficiently,and the decarburizing efficiency is deteriorated. Further, there is nodescription about the manner of blowing of oxygen, which is an importantfactor to enhance the decarburizing efficiency, in the above patentpublication. That is, there is no description of whether the hardblowing operation or the soft blowing operation is conducted.

Japanese Unexamined Patent Publication No. 6-116626 discloses atechnique in which molten steel is refined in a degree of vacuum of 760to 100 Torr while a mixing ratio of top blown oxygen gas and Ar gas ischanged in accordance with the degree of vacuum. There is a descriptionthat the carbon concentration at the start of decarburization is 1.0 to0.1%. This operation is mainly conducted at a high carbon concentration.Even in this case, there is no description about the manner of blowingof oxygen, which is an important factor to enhance the decarburizingefficiency, in the above patent publication. That is, there is nodescription of whether the hard blow operation or the soft blowoperation is conducted. Further, there is no description about theeffective decarburizing condition when pure oxygen gas is used.

In the prior art in which the straight barrel type vacuum refiningapparatus is used, examples are shown in the case of a region in whichthe carbon concentration is high and also in the case in which thedegree of vacuum is too low, wherein the decarburizing principles arequite different from each other. Concerning the oxygen blowingcondition, it is only recognized that the soft blow operation isrequired in the example, and no technical investigation has been madeinto the appropriate oxygen blowing condition.

In the straight barrel type vacuum refining apparatus, the followingoperation is effective. Before blowing oxygen gas into the vacuum vesselfor the purpose of decarburization, in order to raise the temperature ofmolten steel in the vacuum vessel of the refining apparatus, Al alloy isadded to the molten steel. Top blown oxygen is fed onto the surface ofthe molten steel, so that Al is burned to raise the temperature of themolten steel. The aforementioned Al heating is a technique in which Alalloy is continuously added to the molten steel or Al alloy is added tothe molten steel all at once, and during the above Al alloy addingoperation, oxygen is top-blown to the molten metal, so that Al isoxidized and the temperature of molten steel is raised by the heatgenerated by the oxidization of Al. In this case, when carbon containedin the molten steel is oxidized, the amount of oxygen used for oxidizingAl is reduced. Therefore, it is not preferable to oxidize carboncontained in the molten steel. It is necessary to react the top-blownoxygen with Al at a high efficiency. Also, it is necessary to add thethus generated heat to the molten steel at a high efficiency. From theviewpoint of thermodynamics, carbon and Al are respectively oxidized asfollows. When the partial pressure of CO is high, that is, when thevacuum is low, the oxidization of Al occurs prior to the oxidization ofcarbon. However, when the partial pressure of CO is low, that is, whenthe vacuum is high, the oxidization of carbon occurs prior to theoxidization of Al. Consequently, the appropriate degree of vacuum hasnot been known in the actual operation for the following reasons.Although a low vacuum is necessary for suppressing the oxidization ofcarbon, in a free surface region in which the reaction occurs, thetemperature is raised by the reaction, and the partial pressure of CO isnot same as the degree of vacuum.

Further, it is necessary to effectively discharge Al₂ O₃ produced in thereaction outside the vacuum vessel. The reason is described below. Whena large amount of Al₂ O₃ is suspended on the surface of the vacuumvessel, since the heat conduction of Al₂ O₃, which is an oxide, is low,Al₂ O₃ becomes a resistance to heat transfer. Accordingly, thecoefficient of heat transfer on the surface region of the vacuum vesselis deteriorated, so that heat transfer efficiency is lowered. In orderto discharge slag from the vacuum vessel, it is necessary to keep thevacuum vessel in a low degree of vacuum. The reason why the vacuumvessel is kept in a low degree of vacuum condition is described asfollows. When the vacuum vessel is kept in a high degree of vacuum, aninterval between the lower end of the dipping portion and the surface ofthe molten steel in the vacuum vessel is increased, and slag particlesin the molten steel are moved in a stream flowing downward. However,very few of the particles of slag arrive at the lower end of the dippingportion, and most slag particles are circulating in the vacuum vessel.The above slag flow rises to a bubble activating surface being carriedby a rising stream. Therefore, an amount of Al₂ O₃ suspended in thesurface region is accumulated, so that the heat transfer efficiency islowered.

An effective means for discharging Al₂ O₃ from the straight barrel typevacuum refining apparatus has not been found.

In order to effectively transfer the generated heat to the entire moltensteel, it is necessary that an amount of circulating molten steel issufficiently large. In this case, the amount of circulating molten steelmay be smaller than that in the case of blowing oxygen performed for thepurpose of decarburization. The reason is that not only convection heattransmission conducted by a circulating molten steel flow but alsoconduction heat transmission caused by a difference in temperaturecontributes to the heat transfer. However, in the case where the degreeof vacuum is too low, gas blown into the molten steel expands greatlywhen it rises to the surface. Accordingly, the stirring energy isreduced and the molten steel is not agitated and mixed sufficiently. Asa result, the heat transfer efficiency is lowered. Therefore, it isnecessary that the degree of vacuum is maintained at the mostappropriate value.

It is described in Japanese Unexamined Patent Publication No. 58-9914that desulfurization is conducted after the high vacuum treatment ofdecarburization or hydrogen removal in the refining method of moltensteel performed at a reduced pressure. In the above patent publication,a method is disclosed in which powder for refining is blown onto moltensteel in a reduced pressure at a sufficiently high speed so that thepowder can get into the molten steel. According to the above method, aflow speed of gas to be blown to the molten steel must be not lower thanMach 1, that is, when the flow speed of gas is higher than Mach 1, thepowder for refining can get into the molten steel sufficiently.

According to the above method, the flow speed of gas to be blown to themolten steel is very high as described above. Accordingly, the moltensteel splashes, and a lance and refractory material in the vessel aredamaged, and further the metal adheres to the inside of the vessel. Inorder to remove the adhering metal, it takes time and labor. In order toblow the gas at a high flow speed of not less than Mach 1, it isnecessary to reduce the nozzle diameter of the lance. Therefore, when arefining agent is blown into the vacuum vessel by the top-blowing lanceinserted into it, in addition to the usual oxygen blowing hole, it isnecessary to form a new blowing hole exclusively used for blowing therefining agent, which causes a problem with respect to the apparatus. Onthe other hand, when the refining agent is blown by the oxygen blowinglance, it is necessary to feed a large amount of carrier gas to ensurethe blowing speed. As a result, the temperature is lowered, and furtherthe utility cost is increased.

Japanese Unexamined Patent Publications No. 5-287357 and No. 5-171253disclose a method in which an RH type vacuum refining apparatus having avessel bottom is used and powder used for refining is blown from awater-cooled top-blowing lance inserted into a vacuum vessel so as torefine molten steel.

In the above patent publications, the following are described. In orderto enhance the powder trapping efficiency, it is preferable to conduct ahard blow operation. When the hard blow operation is conducted in an RHvacuum refining apparatus, it is necessary to prevent the oxygen jetfrom colliding with the vessel bottom. Therefore, when oxygen gas isblown into the vacuum vessel from the top-blowing lance, it is necessaryto ensure a head of molten steel in accordance with the depth of acavity formed on the molten steel surface. For this reason, when powderfor refining is blown into the vacuum vessel, a high degree of vacuum ofnot more than 100 Torr must be maintained. However, when the vacuumvessel is maintained in a high degree of vacuum condition, the amount ofpowder which is exhausted with the exhaust gas is increased. As aresult, the powder trapping efficiency with respect to molten steel islowered, and the reaction efficiency is deteriorated. In order toenhance the powder trapping efficiency, the blowing speed must beincreased.

Concerning the circulating speed of molten steel in the vessel or ladleof the conventional vacuum refining apparatus, the renewal speed ofmolten steel is not high, so that a high blowing speed is required.However, when a jet speed of carrier gas is increased for the purpose ofincreasing the blowing speed of powder used for refining, the amount offlowing gas is increased and also spitting is increased. Therefore, itis not preferable to increase the jet speed of carrier gas. As isconventionally known, the speed of powder is a half of the speed ofcarrier gas at most, and further it is reported that the depth ofintrusion of powder is constant irrespective of an amount of flowingcarrier gas. For the above reasons, it is not advantageous that thespeed of carrier gas is increased.

An example in which a desulfurizing agent is blown to molten steel in astraight barrel type vacuum refining apparatus is disclosed in JapaneseUnexamined Patent Publication No. 6-212241. However, in the above patentpublication, there is no description about the vacuum and flow speedwhich are important factors to determined the efficiency.

As described above, there is no disclosure of the condition in which thedesulfurizing agent is added to molten steel in the straight barrel typevacuum refining apparatus.

In the refining method of molten steel conducted in a reduced pressure,when the composition of molten steel is adjusted after the process ofdecarburization or the processing in a high degree of vacuum, thetemperature in the vacuum vessel is raised to suppress the adheringmetal. In order to accomplish the above object, the molten steel issubjected to burner heating by using a top-blowing lance, so that thetemperature of molten steel can be raised.

In the above case, since the pressure in the vacuum vessel is reduced,the length of a combustion flame blown out from the top-blowing lancetends to extend. However, when the flame reaches the surface of moltensteel, a combustion improver of hydrocarbon, which has not burned yet,reacts with the molten steel, so that the concentrations of carbon andhydrogen in the molten steel are increased, which causes a seriousproblem. In order to solve the above problem, the degree of vacuum maybe lowered so as to shorten the length of the flame, or an intervalbetween the lance and the molten steel surface may be increased. In thecase of RH, in order to circulate the molten steel, the molten steelmust be sucked up into the vacuum vessel. Therefore, it is impossible toreduce the degree of vacuum. Accordingly, only one method of increasingthe lance height can be adopted. However, according to this method, aninterval between the average flame region and the molten steel surfaceis increased. Therefore, the heat transfer efficiency is lowered.

With regard to the burner heating conducted in a straight barrel typevacuum refining apparatus, there is no specific disclosure.

SUMMARY OF THE INVENTION

An object of the present invention is to solve various problems of theprior art by providing the most appropriate refining condition in avacuum vessel when molten steel is refined for decarburization in astraight barrel type vacuum refining apparatus.

That is, an object of the present invention is to provide the mostappropriate vacuum and oxygen conditions in the vacuum vessel to refinemolten steel.

Another object of the present invention is to provide the mostappropriate Al heating method by which the temperature of molten steelin the vacuum vessel is raised to a predetermined value.

Still another object of the present invention is to provide the mostappropriate desulfurizing condition for molten steel in the vacuumvessel.

Still another object of the present invention is to provide a method ofheating the molten steel in the vacuum vessel and the surface ofrefractory material of the vacuum vessel by means of burner heating.

The above objects of the present invention can be accomplished by thefollowing refining method.

The refining method of the present invention is described as follows.First, molten steel, the carbon content of which has been adjusted to benot more than 0.1% by means of decarburization conducted in a converter,is charged into a vacuum vessel of a straight barrel type vacuumrefining apparatus. While the atmosphere in this vacuum vessel ismaintained in a low degree of vacuum of 105 to 195 Torr, oxygen is blownto the molten steel, from a top-blowing lance, at a blowing speed suchthat the depth of a cavity with respect to the stationary molten steelsurface in the vacuum vessel is 150 to 400 mm.

When the atmosphere in the vacuum vessel is maintained in the low degreeof vacuum described above, it is possible to reduce an interval betweena lower end of the dipping portion of the vacuum vessel and a surface ofthe molten steel in the vacuum vessel. Due to the foregoing, slagparticles in the molten steel on the molten steel surface can be easilydischarged from the lower end of the dipping portion of the vacuumvessel to the outside of the vacuum vessel. As a result, almost all theslag particles existing in the vacuum vessel can be discharged in ashort period of time. Accordingly, iron oxide generated in the processof blowing oxygen by means of top-blowing can exist in the molten steelin the form of pure FeO. Due to the foregoing, the decarburizing oxygenefficiency can be maintained high.

In order to enhance the decarburizing efficiency, it is necessary toraise a temperature in a region (hot spot) where an oxygen jet blown outfrom the top-blowing lance impinges with the surface of molten steel.For this reason, in the present invention, oxygen is blown from thelance in a hard blow condition so that the depth of a cavity is 150 to400 mm. Even when oxygen is blown from the lance in a hard blowcondition as described above, since the atmosphere in the vacuum vesselis in a low vacuum condition as described above, splashing of the metalin the vacuum vessel can be reduced. Accordingly, this method can be putinto practical use.

Next, in the present invention, before the decarburization conducted byblowing oxygen or before the processing conducted in a high vacuum(decarburization or hydrogen removal) or before the compositionadjustment conducted by adding alloy, the atmosphere in the vacuumvessel is maintained in a low degree of vacuum, and Al alloy is chargedinto the vacuum vessel, and then oxygen is fed from the top-blowinglance. In the atmosphere described above, carbon is seldom oxidized.Accordingly, oxygen can be effectively utilized for oxidizing Al, andparticles of Al₂ O₃ can be easily discharged outside the vessel. Inorder to obtain a higher reaction efficiency of Al alloy, it ispreferable to blow oxygen gas from the top-blowing lance in a hard blowcondition so that the cavity depth can be 50 to 400 mm.

Next, in the present invention, before the adjustment of composition byadding alloy conducted after decarburization, the atmosphere in thevacuum vessel is maintained in a low degree of vacuum of 120 to 400Torr, and a desulfurizing agent, the primary component of which is quicklime, is charged from the top-blowing lance into the vacuum vesseltogether with carrier gas. According to the above method, when theconcentration of "T.Fe+MnO" of converter slag outside the vacuum vesselis lowered, the desulfurizing reaction of the molten steel in the vacuumvessel can be facilitated, and further the desulfurizing agent in themolten steel can be easily made to flow out from the vacuum vessel. Dueto the foregoing, the basicity of slag outside the vacuum vessel can beincreased, so that rephosphorization can be prevented. Therefore, thedesulfurizing treatment can be very effectively performed.

Next, in the present invention, while the composition is being adjustedby adding alloy, the atmosphere in the vacuum vessel is maintained in alow degree of vacuum of 100 to 400 Torr, and combustion improving gas ofhydrocarbon such as LPG and oxygen gas are blown out from thetop-blowing lance, so that a burner can be formed and the molten steelis heated by the thus formed burner. In this way, the temperature ofmolten steel can be adjusted and the metal can be prevented fromadhering to the vacuum vessel.

By the above method, it is possible to reduce the height of the lance,so that heat can be highly effectively transferred to the molten steel.Further, when the convection heat transfer is caused as well as theradiation heat transfer, the heat transfer efficiency can be moreenhanced.

It should be noted that the present invention includes a case in whichthe above processes are combined with each other so as to refine moltensteel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional front view of a straight barrel type vacuumrefining apparatus illustrating its general construction in accordancewith the present invention.

FIG. 2 is a graph showing a relation between the degree of vacuum andthe decarburizing oxygen efficiency.

FIG. 3 is a graph showing a relation between the cavity depth and thedecarburizing oxygen efficiency.

FIG. 4 is a graph showing a relation between the degree of vacuum andthe cavity depth, wherein the most appropriate decarburizing conditionis shown.

FIG. 5 is a graph showing a relation between the degree of vacuum andthe heat transfer efficiency of aluminum heating.

FIG. 6 is a graph showing a relation between the degree of vacuum andthe concentration of (T.Fe+MnO).

FIG. 7 is a graph showing a relation between the degree of vacuum andthe processing time in each process.

FIG. 8 is a sectional front view of a conventional RH type vacuumrefining apparatus illustrating its general construction.

BEST MODE FOR CARRYING OUT THE INVENTION

The molten steel refining method of the present invention now will beexplained in detail.

According to the method of the present invention, molten steel subjectedto decarburization by a converter is refined.

In the straight barrel type vacuum refining apparatus used for thepresent invention, no vessel bottom is provided in the molten steeldipping portion of the vacuum vessel. Accordingly, even in a low degreeof vacuum (when the vacuum is measured in the unit Torrs, the Torrnumber is large), it is possible to blow oxygen from a top-blowinglance.

Referring to FIG. 1, the refining apparatus of the invention will beexplained below.

In the drawing, molten steel 2 is held in a ladle 3. A lower portion ofthe cylindrical barrel 7 of the vacuum vessel 1 is dipped in the moltensteel 2, so that a dipping portion 9 can be formed. A ceiling 8 isprovided in the upper portion of the cylindrical barrel 7. A lowerportion of the cylindrical barrel 7 is open. Accordingly, no vesselbottom is provided at the lower portion of the cylindrical barrel 7. Thelower portion of the cylindrical barrel 7 is formed into a cylindricalshape.

In the ceiling 8, there is provided a holding device 10 for holding atop-blowing lance. By this holding device 10, the top-blowing lance 4 isheld and moved upward and downward so that the distance from the lanceto the molten steel surface can be maintained appropriately.

Porous bricks 11 are provided at the bottom of the ladle 3. The porousbricks 11 are arranged at a position distant from the bottom center by adistance K. For example, Ar gas 5-1 is blown toward a space 12 of thecylindrical barrel portion 7 via these porous bricks 11. The position atwhich Ar is blown deviates from the center of the bottom of the ladle.Accordingly, a current of Ar gas deviates from the center, and a bubbleactivating surface is formed in a portion on the surface of moltensteel. In this case, the bubble activating surface is defined as anactivating surface formed when bubbles of a gas, which has been blowninto molten steel, rise and appear on the surface. When Ar gas is blowninto the molten steel while it deviates from the center of the bottom ofthe ladle, one portion of molten steel in the barrel portion is pushedup, and the other portion where Ar gas is not blown is lowered. As aresult, molten steel circulates between the ladle 3 and the cylindricalbarrel 7 of the vacuum vessel.

A current of oxygen gas 5 is jetted into the circulating molten steel 2from the water cooled lance 4 inserted from the ceiling 8 of the vacuumvessel into the vacuum vessel, so that a cavity (recess) 6 is formed onthe surface of molten steel. A slag layer 13 is formed on the surface ofmolten steel between the inner wall of the ladle 3 and the outer wall ofthe dipping portion 9 of the cylindrical barrel portion 7. A vacuumdevice (not shown) is connected with the vacuum vessel 1, and the vacuumof the atmosphere in the space 12 of the barrel portion 7 is adjusted tobe a predetermined value.

The vacuum refining apparatus of this embodiment has a straight barreltype vacuum vessel, the dipping portion of which has no vessel bottom.In the case of refining molten steel, the carbon concentration of whichhas been adjusted to be not more than 0.1% by means of decarburizationconducted in a converter, it is possible to blow oxygen gas even if thedegree of vacuum is low, because the straight barrel type vacuum vesselhas no bottom. When oxygen gas is blown into molten steel by means oftop-blowing in the above apparatus, it is necessary that the blowingoperation is conducted in a low vacuum condition to facilitate thedecarburizing reaction. The decarburizing reaction performed bytop-blown oxygen in a region where the carbon concentration is not morethan 0.1% proceeds in the following manner. Since the carbonconcentration is low, top-blown oxygen temporarily generates iron oxide,and the thus generated iron oxide reacts with carbon contained in moltensteel. Accordingly, in order to make the reaction proceed effectively,the following three factors are important.

(1) Iron oxide, which has been generated on the surface, is dispersedinto fine particles, so that the reacting surface area can be increased.

(2) Iron oxide is made to be pure FeO so as to enhance the activity andensure the reaction property.

(3) Feed of carbon from the molten steel bulk to the reaction site isfacilitated.

Factor (3) is influenced by the stirring and mixing conducted by gasblown to the molten steel from a lower position. When oxygen gas isblown in a high degree of vacuum, bubbles of gas grow while they arerising onto the surface. Therefore, the agitating energy increases. Whenthe degree of vacuum is lower than 195 Torr, the stirring energydecreases, and the molten steel is not stirred and mixed sufficiently,so that the carbon feed speed is lowered when carbon is fed from themolten steel bulk to the reaction site. As a result, the decarburizingefficiency is deteriorated. Also, factor (1) is determined by a relationbetween the impinging surface of top-blown oxygen and the bubbleactivating surface. That is, iron oxide is generated on the impingingsurface of top-blown oxygen. On the other hand, an iron oxide layergenerated on a large bubble activating surface is formed in such amanner that individual bubbles of gas are dispersed into fine particleswhen bubbles of gas blown from a lower position rise and appear on thesurface. Accordingly, it is preferable that an overlapping region of theimpinging surface of top-blown oxygen and the bubble activating surfaceis not less than 50% of the impinging surface of top-blown oxygen.Factor (2) is greatly influenced by the removal property of converterslag mixed into the vacuum vessel before the processing. That is, whenconverter slag exists on the surface of molten steel provided in thevacuum vessel, iron oxide generated in the process of blowing oxygen bymeans of top-blow is mixed with the converter slag, and theconcentration of FeO is remarkably reduced. In this case, the reactingproperty of FeO with C is greatly deteriorated, and the decarburizingefficiency is remarkably lowered. In order to discharge the converterslag from the vacuum vessel, it is necessary to maintain the vacuumvessel in a low degree of vacuum. The reason is described as follows.When the vacuum vessel is maintained in a high degree of vacuum (whenthe vacuum is measured in the unit Torrs, the Torr number is small), aninterval between the lower end of the dipping portion and the surface ofmolten steel in the vacuum vessel is increased, and although slagparticles in the molten steel on the surface are moved downward by beingcarried by a stream of molten steel going downward, few particles reachthe lower end of the dipping portion, and most particles only circulatein the vacuum vessel. The above slag particles rise on the bubbleactivating surface being carried by a stream of molten steel goingupward. Accordingly, the above slag particles are mixed with iron oxidegenerated by top-blown oxygen, so that the concentration of FeO islowered. On the other hand, when the vacuum vessel is maintained in alow vacuum condition, the degree of vacuum of which is not less than 105Torr, the distance between the lower end of the dipping portion and thesurface of molten steel in the vacuum vessel is decreased. Therefore,slag particles in the molten steel on the surface are moved downwardbeing carried by a stream of molten steel going downward, so that theycan be easily made to flow out from the lower end of the dipping portionto the outside of the vacuum vessel. As a result, almost all slag can bedischarged from the vacuum vessel in a short period of time. Therefore,iron oxide generated by top-blown oxygen can remain in the form of pureFeO. Consequently, it is possible to keep the decarburizing oxygenefficiency high.

Due to the foregoing, as shown in FIG. 2, it is possible to obtain adecarburizing oxygen efficiency of not less than 80% in a region wherethe vacuum is 105 to 195 Torr.

It is preferable that a distance N from the lower end of the dippingportion to the surface of molten steel in the vacuum vessel is set at1.2 to 2 m. The above distance 1.2 to 2 m is the condition necessary formaking the oxide generated on the surface of molten steel in the vacuumvessel flow out outside the vessel effectively. When the distance N isshorter than 1.2 m, oxide flows outside the vessel in a short period oftime. Therefore, the residence time (reaction time) in the molten steelis short, and there is a high possibility that the oxide flows outsidethe vessel before the completion of reaction. When the distance N islonger than 2 m, a flow speed of the stream going downward is lowered ata position close to the lower end of the dipping portion. Accordingly,it is difficult for the oxide to flow out from the vacuum vessel.

However, when a reducing speed, i.e., the chemical reaction speed ofiron oxide conducted by top-blown oxygen is low, even if the degree ofvacuum is appropriate, it is difficult to make progress in the reductionof iron oxide, and the decarburizing oxygen efficiency can not beenhanced. Since the reducing reaction speed is substantially determinedby temperature, the temperature in a impinging region (hot spot) inwhich an oxygen jet impinges with molten steel is important, wherein thegenerated iron oxide is mainly reduced in this impinging region.Accordingly, in order to enhance the decarburizing efficiency, it isnecessary to conduct a hard blow operation so as to raise the hot spottemperature. Concerning the condition of the hard blow operation, thedepth of a cavity formed on the molten steel surface by an oxygen jet ismade to be 150 to 400 mm.

As illustrated in FIG. 3, when the cavity depth is not less than 150 mm,the decarburizing oxygen efficiency can be made to be not less than 80%.

The most serious problem caused when oxygen is blown into a low degreeof vacuum atmosphere in the hard blow operation is the occurrence ofsplash. Conventionally, it is considered that the splash of molten steeloccurs when molten steel is dispersed by the kinetic energy of top-blownoxygen gas. Therefore, it is considered that the occurrence of splashcan be prevented only when the kinetic energy of molten steel issuppressed by conducting a very soft blowing operation. Also, it isconsidered that the occurrence of splash can be prevented only when thedispersing direction of splash is changed from the outward to the inwardby extremely increasing the depth of the cavity in a very hard blowoperation. The aforementioned methods are common when molten steel isrefined in a converter. However, the oxygen blowing speed of the presentinvention is much lower than that of refining molten steel in aconverter. Therefore, it is difficult to realize a very hard blowingoperation in the present invention. For this reason, it is consideredthat the occurrence of splash can be avoided only when a very softblowing operation is conducted.

However, the present inventors made an investigation located thebehavior of occurrence of splash when the oxygen blowing speed was low.As a result of the investigation, it was found that it is possible tosuppress the occurrence of splash even if the cavity depth is 150 to 400mm. That is, when the oxygen blowing speed is originally low so that thepossibility of occurrence of splash is low, the amount of splash causedwhen oxygen gas is blown is not influenced by the kinetic energy ofoxygen gas but it is influenced by other factors. The primary cause ofsplash is described as follows. Top-blown oxygen of impinge with moltensteel at the hot spot. At this time, iron oxide particles are generatedat the hot spot. When these iron oxide particles are located below thesurface of molten steel and reacted with carbon in the molten steel, COgas is generated. When CO gas is generated in this way, splash iscaused. In the case of a very soft blowing operation, even if iron oxideparticles are generated at the hot spot on the molten steel surface, thedownward kinetic energy of top-blown oxygen gas is low, so that the ironoxide particles can not intrude into the molten steel, and the reactionoccurs only on the molten steel surface. Therefore, drops of moltensteel are not generated even when CO gas is generated. Conventionally,the refining operation has been carried out in this region.

When a hard blowing operating condition is adopted as compared with theabove operating condition, the iron oxide particles generated at the hotspot intrude into molten steel due to the downward kinetic energy of thetop-blown oxygen gas. Accordingly, CO gas is generated in the moltensteel, and splash occurs. For the reasons described above, it isconsidered that splash occurs when the blowing operating condition isharder than the conventional one.

However, when the operating condition is made to be a hard blowingcondition which is harder than the conventional hard blowing operatingcondition, the heat inputting speed per unit area is increased, and thetemperature at the hot spot is raised. Accordingly, the reducing speedof iron oxide is increased, and iron oxide generated on the surface ofmolten steel at the hot spot is reduced by C! in the molten steel in avery short period of time. Therefore, a steady entrapment of iron oxideinto the molten steel can be avoided. As a result, no CO gas isgenerated in the molten steel, so that the occurrence of splash can bedecreased. Concerning the decrease in splash, the critical condition isthat the cavity depth is not less than 150 mm. When the operatingcondition is made to be a hard blow condition which is harder than theabove condition, drops of molten steel are dispersed by the kineticenergy of top-blown oxygen gas in the same manner as that of refiningoperation conducted in a converter. Therefore, the amount of splashcaused in the refining process is increased. The critical condition isthat the cavity depth is not more than 400 mm.

In other words, an upper limit of the cavity depth by which theoccurrence of splash can be reduced and oxygen gas can be blown stably,the degree of vacuum of which is 105 to 195 Torr, is 400 mm asillustrated in FIG. 4.

Accordingly, in the present invention, the cavity depth is limited to arange from 150 to 400 mm, the degree of vacuum of which is 105 to 195Torr. In this connection, mark ◯ in FIG. 3 represents an example inwhich the degree of vacuum is set at 130 Torr, and mark Δ represents anexample in which the degree of vacuum is set at 170 Torr.

In this case, cavity depth L (mm) is computed by the followingequations.

    L=L.sub.n ·exp(-0.78G/L.sub.n)                    (1)

In the above equation, L_(n) is defined by the following equation.

    L.sub.n =63(F/(n·d.sub.N)).sup.2/3                (2)

where F is a gas feed speed (Nm³ /Hr), n is a number of nozzles, d_(N)is a diameter of the nozzle throat (mm), and G is a distance (mm) fromthe lance end to the surface of molten steel in the vacuum vessel.

In this case, when the cavity depth is smaller than 150 mm, the hot spottemperature is not sufficiently high. Therefore, even if the degree ofvacuum is appropriate and substantially pure iron oxide is generated,the reducing reaction speed is low, so that the decarburizing oxygenefficiency is low. To the contrary, when the cavity depth is larger than400 mm, the kinetic energy of the top-blown oxygen gas is too high.Accordingly, metal is dispersed, that is, splash is caused. Therefore,it is impossible to put this operating condition into practical use.

In the case where ultra low carbon steel is produced in the refiningprocess, after the completion of decarburization conducted by blowingoxygen, the degree of vacuum in the vacuum vessel is enhanced, and therefining process is transferred to the decarburization conducted in ahigh degree of vacuum. The decarburization conducted in a high degree ofvacuum is performed by utilizing a reaction conducted between oxygen andcarbon melted in molten steel. In this case, a reaction on the freesurface exposed to vacuum is important. Accordingly, when the freesurface is covered with slag, the reaction speed is greatly reduced, andfurther slag is explosively scattered by the action of CO gas generatedin accordance with a decrease in pressure. That is, a phenomenon ofbumping is caused, which produces a serious problem in the refiningoperation. In order to avoid the occurrence of the above problem, it isnecessary to discharge the entire slag, the primary component of whichis iron oxide generated in the process of decarburization conducted byblowing oxygen, outside the vacuum vessel before the start of highvacuum treatment. In order to discharge the entire slag outside thevacuum vessel, it is necessary to reduce the dipping depth of thedipping portion by 0.2 H to 0.6 H, wherein H is a distance (dippingdepth) from the lower end of the dipping portion to the surface ofmolten steel outside the vacuum vessel in a period of thedecarburization conducted by blowing oxygen gas. Due to the foregoing,since a static hydraulic pressure (a head) given by the molten steeloutside the vacuum vessel lowers, the slag particles which have arrivedat the lower end of the dipping portion being carried by a stream ofmolten steel going downward, can be more easily discharged outside thevacuum vessel. When the dipping depth is larger than 0.6 H, the dippingdepth momentarily becomes zero in some portions when the surface ofmolten steel outside the vacuum vessel oscillates. Since the outside airis sucked into the vacuum vessel in this case, the concentration ofnitrogen in molten steel is increased. When the dipping depth is smallerthan 0.2 H, the head is not sufficiently low. Therefore, it isimpossible to discharge the entire slag outside.

Next, Al heating of molten steel will be explained as follows.

In order to accomplish Al heating at a high efficiency in which Al addedto molten steel is burned in top-blown oxygen gas so as to raise thetemperature of molten steel, it is necessary to maintain the vacuumvessel in an appropriate degree of vacuum, and it is also necessary toblow oxygen gas by a hard blow operation.

The present inventors made experiments on Al heating to investigate it.As a result of the experiments, as shown in FIG. 6, it was found thatthe heat transfer efficiency of Al heating was not less than 80% whenthe degree of vacuum was maintained in a range from 100 to 300 Torr.

In the case of a high vacuum condition in which the degree of vacuum islower than 100 Torr, the oxidizing reaction of carbon occurs togetherwith the oxidization of Al. Therefore, the utilization efficiency ofoxygen is lowered, and further it is difficult to discharge Al₂ O₃ whichhas been generated in the above oxidizing reaction. Accordingly, theheat transfer efficiency is deteriorated. On the other hand, in the caseof a low vacuum condition in which the degree of vacuum is higher than100 Torr, the decarburizing reaction seldom occurs. Accordingly, theoxygen utilization efficiency is high in the oxidization of Al. Further,since the interval N between the lower end of the dipping portion andthe surface of molten steel in the vacuum vessel becomes small,particles of Al₂ O₃ in molten steel on the surface are moved by acurrent of molten steel going downward, so that they can easily flowoutside the vacuum vessel. Therefore, the heat transfer efficiency canbe maintained high. In the case of a low vacuum condition in which thedegree of vacuum is higher than 300 Torr, the amount of circulatingmolten steel is lowered, so that the heat transfer efficiency isdeteriorated.

It is preferable that the distance N between the lower end of thedipping portion and the surface of molten steel in the vacuum vessel is1.2 to 2 m. The above condition is necessary for making the oxidegenerated on the surface of the vacuum vessel flow outside the vesseleffectively. When the distance N is shorter than 1.2 m, the oxide flowsoutside the vessel in a short period of time. Therefore, the residencetime (reaction time) in molten steel is short, and most of the oxideflows out before the heat of Al₂ O₃ particles is sufficientlytransferred to molten steel. When the distance N is longer than 2 m, aflow speed of the current of molten steel going downward is decreased atthe lower end of the dipping portion. Accordingly, it becomes difficultfor the oxide to flow outside the vessel.

According to the investigation made by the inventors, it was found thata higher reaction efficiency was obtained when a hard blowing operationwas conducted. When oxygen gas is blown by means of top-blowing in theabove appropriate vacuum condition, the oxidizing reaction of Al meltedin the molten steel is conducted in such a manner that a coat of Al₂ O₃is generated on the surface of molten steel with which the top-blownoxygen gas has collided. This coat of Al₂ O₃ is crushed by the downwardkinetic energy of the top-blown oxygen gas and suspended in the moltensteel. However, in the case where the kinetic energy of the top-blownoxygen gas is low, the coat of Al₂ O₃ can not be crushed by thetop-blown oxygen but it is crushed by a current of bottom-blown gaswhich goes upward. Accordingly, the thus crushed Al₂ O₃ is not suspendedin molten steel but it temporarily rises up to the surface of moltensteel. As described above, in the case where the kinetic energy oftop-blown oxygen gas is not sufficiently high, it is difficult for Al₂O₃ to be suspended in molten steel. Accordingly, even if the degree ofvacuum is appropriate, Al₂ O₃ accumulates on the surface, and the heattransfer efficiency is lowered. For the above reasons, the downwardkinetic energy of top-blown oxygen gas must be sufficiently high to forma cavity, the depth of which is 50 to 400 mm, on the surface of moltensteel by the oxygen jet. In this case, the cavity depth L (mm) iscomputed by the above equations (1) and (2).

When the cavity depth is larger than 400 mm, the kinetic energy oftop-blown oxygen gas becomes too high, so that the amount of splash isincreased. Accordingly, a cavity depth larger than 400 mm is notappropriate for practical use.

In the case of refining a ultra low carbon steel or in the case ofconducting hydrogen removal, after Al heating has been completed, thedegree of vacuum is increased, and decarburization and hydrogen removalare conducted in a high vacuum condition. Decarburization is conductedin a high vacuum condition by utilizing a reaction of oxygen melted inmolten steel with carbon. Hydrogen removal is also conducted byutilizing a reaction of hydrogen melted in molten steel. Therefore, areaction conducted on the free surface exposed to the vacuum isimportant. Accordingly, when the free surface is coated with slag, thereaction speed is greatly reduced, and further slag is explosivelyscattered by the action of CO gas generated in accordance with adecrease in pressure. That is, a phenomenon of bumping is caused, whichcauses a serious problem in the refining operation. In order to avoidthe occurrence of the above problems, it is necessary to discharge theentire slag completely, the primary component of which is Al₂ O₃generated in the process of Al heating, outside the vacuum vessel beforethe start of decarburization refining and high vacuum processing. Inorder to discharge the entire slag outside the vacuum vessel, it isnecessary to reduce the dipping depth of the dipping portion by 0.2 H to0.6 H, in a period of Al heating for the same reason as that of refininga ultra low carbon steel. In this way, the entire slag can be easilydischarged outside the vacuum vessel.

Next, a method of desulfurization conducted in a reduced pressure willbe explained below.

Concerning the desulfurizing reaction, the deoxidizing reactionconducted by a desulfurizing agent added into the vacuum vessel must beconsidered, and at the same time, the sulfurizing reaction conductedwhen oxygen is fed from converter slag, the iron oxide concentration ofwhich is high, must be considered. That is, since the desulfurizingreaction formula can be described as S!+CaO CaS+ O!, in order to makethe desulfurizing processing proceed effectively, it is indispensable tosufficiently lower the concentration of O! expressed on the right side.In order to make the desulfurization processing proceed effectively, inthe process of deoxidation conducted before the desulfurizationprocessing, it is important to sufficiently lower the oxygen potential(T.Fe+MnO) in the converter slag outside the vacuum vessel. However,when the oxygen potential in the converter slag is sufficiently lowered,phosphorus oxide contained in the converter slag becomes unstable in theprocess of desulfurization, so that the concentration of phosphorus inmolten steel is increased, that is, a phenomenon of rephosphorizationreaction occurs. In order to suppress the occurrence of therephosphorization reaction, it is necessary to increase theconcentration of CaO in the converter slag outside the vacuum vessel,the oxygen potential of which is lowered in the process ofdesulfurization, so that the basicity of the converter slag can beenhanced and the phosphorus oxide can be stabilized even if the oxygenpotential is low.

That is, in order to conduct the desulfurization effectively andsuppress the rephosphorizing reaction, the following two factors arerequired.

(1) Concerning the converter slag outside the vacuum vessel, theconcentration of (T.Fe+MnO) is sufficiently lowered in the process ofdeoxidation.

(2) Concerning the converter slag outside the vacuum vessel, thebasicity is enhanced in the process of desulfurization.

The above two conditions can be satisfied when the vacuum is kept at 120Torr. That is, when the vacuum is low, a distance between the lower endof the dipping portion and the surface of molten steel in the vacuumvessel is decreased. Therefore, the following two characteristics areexhibited.

(A) When gas is blown from a lower position into the vacuum vessel, awave motion on the surface of molten steel in the vacuum vessel can beeasily transmitted to the molten steel outside the vacuum vessel.

(B) After a desulfurizing agent, the principal component of which isquick lime fed onto the surface of molten steel in the vacuum vessel,has been suspended into the molten steel, it can be easily made to flowout from the lower end of the dipping portion to the outside of thevacuum vessel. In this case, characteristic (A) greatly affects thefactor (1) described before. Since the molten steel outside the vacuumvessel is also agitated, the reaction speed of Al melted in the moltensteel with the slag outside the vacuum vessel is increased. Accordingly,the concentration (T.Fe+MnO) of the converter slag outside the vacuumvessel is effectively lowered to a value not more than 5% in a shortperiod of time as illustrated in FIG. 6.

On the other hand, in the case of a high vacuum condition in which thedegree of vacuum is lower than 120 Torr, the molten steel outside thevacuum vessel seldom flows so that the stirring can not be conductedstrongly, and Al melted in the molten steel seldom reacts with the slagoutside the vacuum vessel. Characteristic (B) considerably affects thefactor (2). That is, during the desulfurizing processing, adesulfurizing agent, the principal component of which is quick lime fedonto the molten steel surface in the vacuum vessel, flows out from thelower end of the dipping portion to the outside of the vacuum vesselbeing carried by a current of molten steel going downward. Accordingly,the basicity of the slag outside the vacuum vessel is increased inaccordance with the progress of processing. Therefore, therephosphorization reaction can be prevented. On the other hand, in thecase of a high vacuum condition in which the degree of vacuum is lowerthan 120 Torr, the desulfurizing agent seldom flows outside the vacuumvessel. Therefore, the basicity of the slag outside the vacuum vessel isnot raised, and the rephosphorization reaction can not be avoided.

In the case of a low vacuum condition in which the degree of vacuum ishigher than 400 Torr, bubbles of gas blown into the molten steel blow upgreatly, so that the stirring energy is decreased. Accordingly, themolten steel is not stirred and mixed sufficiently, and thedesulfurizing efficiency is deteriorated.

Next, the present inventors made experiments in which a straight barreltype vacuum refining apparatus was used as follows. Under the conditionthat the renewal speed of molten steel was sufficiently high at theblowing position, powder for refining was blown to molten steel. Inorder to obtain the most appropriate blowing condition so that a highreacting efficiency can be easily provided, a lance of large diameter,which had already been established, was commonly used to blow powder forrefining, and blowing was conducted in a low vacuum condition at a lowblowing speed. As a result of the above experiments, the following werefound. When the renewal speed of molten steel was sufficiently high onthe blowing surface and the vacuum condition was low, even if theblowing speed was low, it was possible to obtain a high efficiency oftrapping powder and the reaction efficiency was enhanced.

According to the present invention, when the straight barrel type vacuumrefining apparatus was used, even in a low vacuum condition in which thedegree of vacuum was not less than 120 Torr, it was possible to ensurean activating effect on the molten steel surface provided by thecirculating gas sent from the ladle bottom, and it was also possible toensure a large amount of circulating molten steel. Accordingly, even ifthe blowing speed of oxygen gas was low, it was possible to obtain ahigh rate of trapping powder. Specifically, the vacuum refiningapparatus was used, and the blowing speed was set in a range from 10m/sec to Mach 1 in a low vacuum condition in which the degree of vacuumwas not less than 120 Torr. In the above operating condition, it waspossible to provide a high powder trapping rate.

According to the present invention, the cavity on the molten steelsurface was formed when oxygen gas was blown at a blowing speed of 10m/sec which was the minimum value necessary for trapping powder used forrefining. When powder for refining was blown into molten steel at thisspeed, the amount of powder for refining sucked uselessly into theexhaust gas system was decreased, and it was possible to blow powder forrefining into molten steel at a high solid-gas ratio from a commonlance.

The depth of intrusion of powder for refining, which was blown to moltensteel, is substantially constant irrespective of the flow rate ofcarrier gas. Accordingly, it is sufficient that the blowing speed ofpowder for refining is set at the minimum speed by which powder forrefining can be sent to a position immediately below the molten steelsurface. Although the minimum speed is somewhat different according tothe blowing condition, as a result of experiments, it was necessary tomaintain the speed at a value not less than 10 m/sec. It was notpreferable that the blowing speed was set at a value not less than Mach1, because molten steel splashed and further the temperature of moltensteel dropped.

In the present invention, a straight barrel type vacuum refiningapparatus is used. Accordingly, a head of molten steel in the vacuumvessel can be maintained at a sufficiently high value even in a lowvacuum condition of not less than 120 Torr. When a large amount of gasis blown from the ladle bottom, the renewal speed on the surface ofmolten steel in the vacuum vessel is much faster than that of a commondegasifying ladle device. For example, when the degree of vacuum is 150Torr, a difference of the head of molten steel between the inside andthe outside of the vacuum vessel is 1.1 m. When the amount ofcirculating gas sent from the ladle bottom is set at the same value, therenewal speed on the molten steel surface and the circulating speed ofmolten steel are approximately the same as those in the case of blowinggas in a high vacuum condition. Therefore, even in a low vacuumcondition, powder for refining used as a desulfurizing agent, which hasbeen blown into molten steel, can deeply intrude into molten steel inthe ladle being carried by this circulating current, so that thereacting efficiency can be enhanced. Since the straight barrel typerefining apparatus has no vessel bottom, even in a low vacuum condition,no oxygen gas collides with a barrel bottom unlike the RH type refiningapparatus. Accordingly, there is no possibility of damage of refractorymaterial of the vessel bottom.

A molten steel surface arrival speed of carrier gas is computed by thefollowing method.

The Mach number M' in the case of blowing gas from a nozzle is definedby the following equation, where the degree of vacuum is P (Torr) andthe back pressure of carrier gas is P' (kgf/cm²). In the followingequation, M' exists as an implicit function. Therefore, it is computedas a numerical solution. ##EQU1##

The Mach number M at the time of arrival on the molten steel surface canbe computed by the following equation, where G (mm) is a distance fromthe nozzle end to the molten steel surface in the vacuum vessel, do is adiameter of the nozzle exit, and n is a number of nozzles.

    M=6.3M'/(G/{(n·do.sup.2).sup.1/2 })               (4)

The Mach number M is converted into the flow speed U (m/s) at the timeof arrival on the molten steel surface by the following equation.

    U=M×320×0.07p.sup.1/2                          (5)

It is preferable that the distance N from the lower end of the dippingportion to the molten steel surface in the vacuum vessel is set at 1.2to 2 m. This condition is necessary to make a desulfurizing agent fedonto the molten steel surface in the vacuum vessel effectively flowoutside the vessel. When the distance N is shorter than 1.2 m, thedesulfurizing agent flows outside the vessel in a short period of time.Therefore, the residence time (reaction time) is short, and most of thedesulfurizing agent flows outside before the completion of reaction.When the distance N is longer than 2 m, the flow speed of a current ofmolten steel going downward is lowered at the lower end of the dippingportion. Accordingly, it is difficult for the desulfurizing agent toflow outside.

The desulfurizing efficiency (X) can be found by the following equation.##EQU2## where S!₁ is a concentration S! (ppm) before processing, andS!₂ is a concentration S! (ppm) after processing.

Next, the operation of burner heating conducted when molten steel isrefined in the straight barrel type vacuum refining apparatus will beexplained. In the burner heating after the completion of decarburizingprocessing or high vacuum processing (including desulfurizingprocessing), oxygen gas and a combustion improving gas of a hydrocarbon,such as LNG, are jetted out onto the molten steel surface from atop-blowing lance, so that the molten steel and the vacuum vessel can beheated.

In the burner heating described above, while the atmosphere in thevacuum vessel is maintained in a low vacuum condition of 100 to 400 Torrand a distance from the end of the lance to the molten steel surface inthe vacuum vessel is adjusted in a range from 3.5 to 9.5 m, theaforementioned combustion gas is blown onto the molten steel surface.

Even in the low vacuum condition described above, when the refiningapparatus of the present invention is used, molten steel can besufficiently stirred and mixed. Accordingly, it is possible to heat themolten steel while the lance height is kept low as described above.Therefore, it is possible to provide a high heat transfer efficiency.According to the prior art, when the degree of vacuum is higher thanthat of the present invention, only radiation heat transfer occurs. Onthe other hand, according to the present invention, not only radiationheat transfer but also convection heat transfer occurs. Therefore, theheat transfer efficiency can be further enhanced.

In a low vacuum condition in which the degree of vacuum exceeds 400Torr, bubbles of gas blown into molten steel expand greatly.Accordingly, the stirring energy is decreased. Due to the foregoing, themolten steel can not be stirred and mixed sufficiently, and the heattransfer efficiency is lowered.

As described above, the characteristic of the present invention can besummarized as follows. In a straight barrel type vacuum refiningapparatus, in an atmosphere of a low vacuum condition of 100 to 400Torr, oxygen gas is blown onto the surface of molten steel by means oftop-blowing in an oxygen blowing condition appropriate for eachprocessing. In this case, the oxygen blowing condition is represented bythe depth of a cavity formed in the molten steel. The objects of blowingoxygen gas in this vacuum vessel by means of top-blowing are describedas follows. The first object is "decarburization" in which oxygen gas isreacted with carbon contained in the molten steel when oxygen gas isblown. The second object is "Al heating" in which the temperature ofmolten steel is raised when Al added to molten steel is burned by oxygengas blown into the molten steel by means of top-blowing. The thirdobject is "desulfurization" in which a flux, such as lime, is addedtogether with carrier gas. The fourth object is "burner heating" inwhich oxygen gas and combustion improving gas of hydrocarbon, such asLNG, are blown by means of top-blowing so as to heat the vacuum vesseland suppress the adhering metal.

FIG. 7 is a graph showing the combination of each processing describedabove. In FIG. 7, each processing is expressed by the processing timeand the vacuum. In the actual operation, each processing isappropriately combined if necessary.

EXAMPLES Example 1

In Example 1, while the straight barrel type vacuum refining apparatusshown in FIG. 1 was used, the decarburizing operation was carried out bymeans of top-blowing. In this case, the capacity of a ladle was 350 ton,the inner diameter D of the ladle was 4400 mm, the diameter d of adipping portion of the vacuum vessel was 2250 mm, the eccentric distanceK of a porous plug from a center of the ladle was 610 mm, and the throatdiameter of a top-blowing lance was 31 mm. Concerning the operatingcondition, the distance G from the lance to the molten steel surface wasset at 3.5 m, and the oxygen blowing speed was set at 3300 Nm³ /h. Underthe above condition, oxygen blowing was carried out for 2 minutes after2 minutes had passed from the start of processing. Due to the aboveoperation, the concentration of carbon was lowered from 450 ppm to 150ppm. After that, degassing processing was carried out. In thisoperation, the depth L of a cavity formed in the process of blowingoxygen gas was 205 mm. A flow rate of Ar gas blown by means ofbottom-blowing was 1000 Nl/min which was maintained constant. The degreeof vacuum at the start of blowing oxygen gas was 165 Torr, and thedegree of vacuum at the end of blowing oxygen gas was 140 Torr. At thistime, the distance N from the lower end of the dipping portion to thesurface of molten steel in the vacuum vessel was 1750 mm, and the depthH of the dipping portion of the vacuum vessel was 450 mm.

As a result of the above operation, the decarburizing oxygen efficiencyη was raised to 85%, and there was no adhering metal.

After the above operation, the vacuum vessel was raised and its dippingdepth H was set at 230 mm. Then the molten steel was stirred for 2minutes to further conduct a decarburizing processing in a high vacuumcondition. Due to the above processing, as compared with a case in whichthe dipping depth H was 450 mm, it was possible to shorten theprocessing time to lower the carbon concentration to 20 ppm by 3minutes. Next, under the operating condition shown on the first table,the operation was carried out. In this case, as a common condition, theoxygen gas blowing speed was set at 3000 Nm³ /h, and the blowing timewas set at 2 minutes. The result of the operation is shown in Table 1.

                                      TABLE 1    __________________________________________________________________________    Degree of    vacuum at       Carbon Carbon    the start       concentration                           concentration    of blowing  Cavity                    before blowing                           after blowing    oxygen gas  depth                    oxygen oxygen η                                     Adhering    (Torr)      (mm)                    (ppm)  (ppm)  (%)                                     metal Evaluation    __________________________________________________________________________    Inventive          165   205 485    127    83.6                                     Zero  ⊚    Example          140   220 479    110    86.0                                     Zero  ⊚          180   120 456    108    81.2                                     Zero  ⊚          120   360 458     97    84.3                                     Zero  ⊚          135   280 444     92    82.2                                     Zero  ⊚          105   215 491    120    86.5                                     Approximate                                           ∘                                     zero          195   150 465    137    76.5                                     Zero  ∘          160   400 483    110    87.1                                     Approximate                                           ∘                                     zero    Comparative           260* 195 445    262    42.7                                     Zero  x    Example           75*  245 458     92    47.1                                     A large                                           x                                     amount of                                     adhesion          125    35*                    482    321    37.6                                     Zero  x          145    460*                    476    107    86.1                                     A large                                           x                                     amount of                                     adhesion    __________________________________________________________________________     Remark: Mark * represents a value outside the range of the present     invention.

As can be seen in Table 1, in the example of the present invention, thedecarburizing oxygen efficiency η was approximately not less than 80%.That is, it was possible to obtain a high decarburizing oxygenefficiency η, and further there was no adhering metal. On the otherhand, in the comparative example, even if the cavity depth wasappropriate, when the degree of vacuum at the start of blowing oxygenwas too low, although there was no adhesion of base metal, thedecarburizing oxygen efficiency η was only a half of that of the presentinvention. When the degree of vacuum was too high, the decarburizingoxygen efficiency η was deteriorated. That is, the decarburizing oxygenefficiency η was not more than 50%, and there was a large amount ofadhering metal.

Even if the vacuum at the start of blowing oxygen was appropriate, whenthe cavity depth was too small, although there was no adhering metal,the decarburizing oxygen efficiency η was very low. When the cavitydepth was too large, although the decarburizing oxygen efficiency η wasnot less than 80%, there was a large amount adhering metal.

Example 2

In Example 2, while the straight barrel type vacuum refining apparatusshown in FIG. 1 was used, decarburizing operation was carried in whichAl heating operation and high vacuum degassing operation were conducted.In this case, the specification of the refining apparatus was the sameas that of Example 1.

Concerning the operating condition, the distance G from the lance to themolten steel surface was set at 3.5 m, and the dipping depth H of thevacuum vessel was set at 450 mm. In the above operating condition,oxygen gas was blown to molten steel at a flow rate of 3300 Nm³ /h afterone minute had passed from the start of processing. Blowing of oxygengas was continued for 6 minutes. Depth L of the cavity formed at thistime was 205 mm. During the oxygen blowing operation conducted over aperiod of 6 minutes, Al was charged every one minute, that is, Al wasequally charged 5 times. In this case, the amount of Al charged in thisway was 460 kg in total. As a result, the molten steel temperature wasraised by 40° C. After that, the degassing processing was carried out inan atmosphere, the degree of vacuum of which was 1.5 Torr. An amount ofbottom-blown Ar was maintained constant at 1000 Nl/min, and the degreeof vacuum was 280 Torr at the start of blowing oxygen and 150 Torr atthe end of blowing oxygen.

As a result of the above operation, the heat transfer efficiency ζ of Alheating was 98.9%, and there was no adhering metal. After the aboveprocessing, the high vacuum degassing processing was carried out. Beforethe high vacuum degassing processing, the carbon concentration was 450ppm, and after the high vacuum degassing processing, the carbonconcentration was decreased to 15 ppm.

After the completion of the above operation, the vacuum vessel wasraised, so that the dipping depth H was set at 230 mm. Then, the moltensteel was stirred for 2 seconds and the decarburizing processing wasfurther conducted in a high vacuum condition. Due to the aboveprocessing, as compared with a case in which processing was conductedunder the condition that the dipping depth H of the vacuum vessel wasset at 450 mm, the processing time necessary for lowering the carbonconcentration to 20 ppm was shortened by 4 minutes.

Next, refining was carried out under the operating condition shown inTable 2. In this case, the common condition is described below. Theamount of charged Al is 460 kg, a flow rate of oxygen gas is 3000 Nm³/h, and a period of time in which oxygen gas is blown is 6 minutes.

The result is shown in Table 2.

                                      TABLE 2    __________________________________________________________________________    Degree    of vacuum       Molten steel                           Molten steel    at the start    temperature                           temperature    of blowing  Cavity                    before blowing                           after blowing                                 Temperature    oxygen gas  depth                    oxygen oxygen                                 rise  ζ                                          Adhering    (Torr)      (mm)                    (° C.)                           (° C.)                                 (° C.)                                       (%)                                          metal Evaluation    __________________________________________________________________________    Inventive          165   230 1605   1647  42    99.4                                          Zero  ⊚    Example          240   205 1612   1654  42    98.7                                          Zero  ⊚          290   315 1597   1639  42    94.6                                          Zero  ∘          105   190 1614   1657  43    99.5                                          Approximate                                                ∘                                          zero          240    50 1589   1629  39    93.4                                          Zero  ∘          200   400 1607   1649  42    99.2                                          Approximate                                                ∘                                          zero    Comparative           60*  245 1611   1653  42    65.9                                          A large                                                x    Example                               amount of                                          adhesion          380    30*                    1604   1632  28    64.7                                          Zero  x          260    550*                    1592   1634  42    99.1                                          A large                                                x                                          amount of                                          adhesion    __________________________________________________________________________     Remark: Mark * represents a value outside the range of the present     invention.

As can be seen on the second table, in the example of the presentinvention, the heat transfer efficiency ζ of Al heating was not lessthan 90%, and there was no adhering metal. However, in the comparativeexample, the degree of vacuum at the start of blowing oxygen gas was toohigh, the heat transfer efficiency ζ of Al heating was lower than 70%,and further there was a large amount of adhering metal. Even if thedegree of vacuum at the start of blowing oxygen was appropriate, whenthe cavity depth was too small, although there was no adhering metal,the efficiency ζ was low. When the cavity depth was too large, althoughthe efficiency ζ was not less than 90%, there was a large amount ofadhering metal

Example 3

While the straight barrel type vacuum refining apparatus shown in FIG. 1was used, molten steel refined by a converter was subjected todecarburization, and then Al was charged into the molten steel toconduct deoxidation, and the desulfurizing operation was carried out. Inthis case, the specification of the refining apparatus was the same asthat of Example 1 except for the diameter (109 mm) of the outlet of thetop-blowing lance.

Concerning the operating condition, the degree of vacuum was set at 200Torr, and the distance G from the lance to the molten steel surface wasset at 2 m, and a desulfurizing agent in which CaF₂ was mixed with CaOby 20% was blown to molten steel for 30 seconds at a speed of 0.4kg/min/t together with carrier gas (Ar), the flow rate of which was 300Nm³ /Hr. Due to the foregoing, the desulfurizing efficiency λ found bythe equation (6) was 0.37. At this time, the back pressure was 4kgf/cm², and the flow speed U at which gas arrived on the molten steelsurface was 193 m/s (the Mach number was 0.62).

Next, the desulfurizing operation was carried out under the operatingcondition shown in Table 3. The result is shown in Table 3.

                  TABLE 3    ______________________________________                Flow           Flow speed at    Degree of   rate           which oxygen    vacuum      of     Number  gas arrives on    during      gas    of Mach the molten    treatment   Nm.sup.3 /                       Number  steel surface Evalua-    Torr        Hr     M       m/s      λ                                             tion    ______________________________________    Inventive           180      300    0.65  195      0.34 ⊚    Example           130      300    0.70  180      0.36 ⊚           270      300    0.59  217      0.35 ⊚           140       5     0.11   29      0.32 ⊚    Com-    95*     300    0.74  162      0.22 x    parative            420*    300    0.55  253      0.25 x    Example           125       1     0.03    7*     0.19 x    ______________________________________     Remark: Mark * represents a value outside the range of the present     invention.

As can be seen in Table 3, it was possible to obtain a highdesulfurizing efficiency λ of not less than 0.30 in any case. However,in the comparative example, unless the degree of vacuum when treatmentis carried out is in the range of the present invention, λ is low, andwhen the flow rate of gas is low and the gas speed at which gas arriveson the molten steel surface is lower than 10 m/s, the efficiency λ isremarkably deteriorated.

Example 4

While the straight barrel type vacuum refining apparatus shown in FIG. 1was used, the molten steel heating operation was carried out. In thisexample, the specification of the refining apparatus was the same asthat of Example 1. Concerning the operating condition, the degree ofvacuum was maintained at 120 Torr, and distance G from the lance to themolten steel surface was set at 4 m. The flow rate of LPG was 120 Nm³/h, and the flow rate of oxygen was 120 Nm³ /h. The heating operationwas carried out for 10 minutes after a period of time of 6 minutes hadpassed from the start of the processing. In this example, the flow rateof Ar blown out by means of bottom-blowing was maintained constant at1000 Nl/min. Due to the foregoing operation, the temperature was raisedby 20° C. compared with a case in which the molten steel heatingoperation was not carried out.

Example 5

Using the straight barrel type vacuum refining apparatus shown in FIG.1, the following processing was carried out to process ultra low carbonsteel. Molten steel in the vacuum vessel of the above refining apparatuswas subjected to Al heating. Then, it was subjected to decarburizationby blowing oxygen gas. After that, while the vacuum vessel wasmaintained in a high vacuum condition, refining of the molten steel wascarried out. Finally, burner heating was conducted on the molten steel.

The specification of the refining apparatus was the same as that ofExample 1 except for the outlet diameter of the top-blowing lance, whichwas 110 mm in this example.

Concerning the condition of Al heating, the degree of vacuum wasmaintained at 250 Torr, and the distance G from the lance to the moltensteel surface was set at 3500 mm. Oxygen blowing was conducted at a flowrate of 3300 Nm³ /Hr for 4 minutes after one minutes had passed from thestart of discharging gas to attain the vacuum condition. At this time,the cavity depth L was 205 mm, the distance N from the lower end of thedipping portion to the molten steel surface in the vacuum vessel was1400 mm, and the distance (dipping depth) from the lower end of thedipping portion to the molten steel surface outside the vacuum vesselwas 450 mm. The flow rate of Ar of bottom-blow was 500 Nl/min. Duringthe oxygen blowing operation conducted over a period of 4 minutes, Alwas charged every one minute. In this case, an amount of Al charged inthis way was 450 kg in total. As a result, the molten steel temperaturewas raised by 40° C. at the heat transfer efficiency of 98.2%.

After that, the distance H was set at 230 mm, and the flow rate of Arwas increased to 750 Nl/min, and molten steel was stirred for 1.5 min,so that slag of Al₂ O₃ in the vacuum vessel was made to flow outside thevacuum vessel completely.

Successively, the degree of vacuum was set at 170 Torr, and oxygen gaswas blown into the molten steel for the purpose of decarburization for 3minutes. In this case, the distance G from the lance to the molten steelsurface was 3500 mm, and the flow rate of oxygen gas was 3300 Nm³ /Hr.In the above operation, the cavity depth L was 205 mm, the distance Nwas 1500 mm, and the distance H was 450 mm. While the flow rate of Ar ofbottom-blowing was set at 700 Nl/min, the carbon concentration waslowered to a value from 430 to 140 ppm. In this case, thedecarburization oxygen efficiency was 85%.

After that, the degree of vacuum was raised to 1 Torr, and oxygen gaswas blown into the molten steel for producing ultra low carbon steel.

After the carbon concentration had reached 20 ppm by the aboveprocessing, the degree of vacuum was returned to 200 Torr, and alloy wasadded to molten steel for the adjustment of composition while burnerheating was being conducted. In this case, burner heating was conductedfor 5 minutes under the following condition. The distance G was set at4500 mm, the flow rate of LPG was 120 Nm³ /Hr, and the flow rate ofoxygen gas was 120 Nm³ /Hr. As a result, the temperature of molten steelwas decreased only by 2° C. during the adjustment of composition.

Example 6

Using a straight barrel type vacuum refining apparatus, thespecification of which was the same as that of Example 5, ultra lowcarbon steel was treated in the following manner. Molten steel in thevacuum vessel of the above apparatus was subjected to Al heating,decarburization conducted by blowing oxygen gas, degassing treatment ina high vacuum condition, deoxidation and desulfurization, and burnerheating.

Al heating was carried out in a degree of vacuum of 250 Torr for 4minutes after one minute had passed from the start of discharging gas toattain the vacuum condition, while the distance G from the lance to themolten steel surface was set at 3.5 m and the flow rate of oxygen gaswas set at 3300 Nm³ /Hr. In this treatment, the cavity depth L was 205mm, the distance N from the lower end of the dipping portion to themolten steel surface in the vacuum vessel was 1400 mm, and the distance(dipping depth) H from the lower end of the dipping portion to themolten steel surface outside the vacuum vessel was 450 mm. The flow rateof Ar of bottom-blow was 500 Nl/min, and Al was charged every one minutein the gas blowing and heating treatments for 4 minutes. The amount ofAl charged in this process was 450 kg in total. As a result, thetemperature of molten steel was raised by 40° C. at the heat transferefficiency of 98.2%.

After that, the distance H was set at 230 mm, and the flow rate of Arwas increased to 750 Nl/min. Then, the molten steel was stirred for 1.5min, so that slag of Al₂ O₃ in the vacuum vessel was made to flowcompletely outside the vessel.

Successively, the degree of vacuum was set at 170 Torr, and oxygen gaswas blown into molten steel for the purpose of decarburization for 3minutes. In this case, the distance G from the lance to the molten steelsurface was set at 3500 mm, and the flow rate of oxygen gas was 3300 Nm³/Hr. In the above operation, the cavity depth L was 205 mm, the distanceN from the lower end of the dipping portion to the molten steel surfacein the vacuum vessel was 1500 mm, and the distance H (dipping depth)from the lower end of the dipping portion to the molten steel outsidethe vacuum vessel was 450 mm. While the flow rate of bottom-blown Ar wasset at 700 Nl/min, the carbon concentration was lowered to a value from430 to 140 ppm. In this case, the decarburizing oxygen efficiency was85%.

After that, the degree of vacuum was raised to 1 Torr, and oxygen gaswas blown into the molten steel to produce ultra low carbon steel.

After the carbon concentration had reached 20 ppm by the aboveprocessing, the molten steel was subjected to deoxidation by adding Al,and the degree of vacuum was returned to 200 Torr and the distance G wasset at 2000 mm. In the above condition, a desulfurizing agent in whichCaF₂ was mixed with CaO by 20% was blown for 30 seconds at a flow rateof 0.4 kg/t/min. Ar carrier gas was fed at 300 Nm³ /Hr, however, themolten steel surface arrival speed of carrier gas Ar was Mach 0.62 (192m/sec). Although the distance N was 1500 mm, the desulfurizingefficiency was 0.35 and rephosphorization did not occur.

After the sulfur concentration had reached 15 ppm by the abovetreatment, the degree of vacuum was maintained at 200 Torr, and alloywas added to molten steel for the adjustment of composition while burnerheating was being conducted. In this case, burner heating was conductedfor 5 minutes under the following condition. The distance G was set at4500 mm, the flow rate of LPG was 120 Nm³ /Hr, and the flow rate ofoxygen gas was 120 Nm³ /Hr. As a result, the temperature of molten steelwas decreased only by 2° C. during the adjustment of composition.

Example 7

Using a straight barrel type vacuum refining apparatus, thespecification of which was the same as that of Example 5, ultra lowsulfur steel having low hydrogen was processed in the following manner.Molten steel in the vacuum vessel of the above apparatus, the carboncontent of which was adjusted to 0.35% in the process of refining in aconverter, was subjected to Al heating, degassing treatment in a highvacuum condition, deoxidation and desulfurization, and burner heating.

Al heating was carried out in a degree of vacuum of 250 Torr for 4minutes after one minute had passed from the start of discharging gas toattain the vacuum condition, while the distance G from the lance to themolten steel surface was set at 3500 mm and the flow rate of oxygen gaswas set at 3300 Nm³ /Hr. In this operation, the cavity depth L was 205mm, the distance N from the lower end of the dipping portion to themolten steel surface in the vacuum vessel was 1400 mm, and the distance(dipping depth) H from the lower end of the dipping portion to themolten steel surface outside the vacuum vessel was 450 mm. The flow rateof Ar of bottom-blow was 500 Nl/min, and Al was charged every one minutein the heating process for 4 minutes. The amount of Al charged in thisprocess was 450 kg in total. As a result, the temperature of moltensteel was raised by 40° C. at the heat transfer efficiency of 98.2%.

After that, the distance H was set at 230 mm, and the flow rate of Arwas increased to 750 Nl/min. Then, the molten steel was stirred for 1.5min, so that slag of Al₂ O₃ in the vacuum vessel was made to flowcompletely outside the vessel.

After that, the degree of vacuum was increased to 1 Torr, and thehydrogen removal treatment was carried out.

After the hydrogen concentration had reached 1.5 ppm by the abovetreatment, the molten steel was subjected to deoxidation by adding Al,and the degree of vacuum was returned to 200 Torr and the distance G wasset at 2000 mm. In the above condition, a desulfurizing agent in whichCaF₂ was mixed with CaO by 20% was blown for 30 seconds at a flow rateof 0.4 kg/t/min. Ar carrier gas was fed at 300 Nm³ /Hr, however, themolten steel surface arrival speed of carrier gas Ar was Mach 0.62 (192m/sec). Although the distance N was 1500 mm, the desulfurizingefficiency was 0.35 and rephosphorization did not occur.

After the sulfur concentration had reached 15 ppm by the abovetreatment, the degree of vacuum was maintained at 200 Torr, and alloywas added to molten steel for the adjustment of composition while burnerheating was being conducted. In this case, burner heating was conductedfor 5 minutes under the following condition. The distance G was set at4.5 m, the flow rate of LPG was 120 Nm³ /Hr, and the flow rate of oxygengas was 120 Nm³ /Hr. As a result, the temperature of molten steel wasdecreased only by 2° C. during the adjustment of composition.

Example 8

Using a straight barrel type vacuum refining apparatus, thespecification of which was the same as that of Example 5, low carbonsteel was treated in the following manner. Molten steel in the vacuumvessel of the above apparatus, the carbon content of which was adjustedto 725 ppm in the process of refining in a converter, was subjected toAl heating, decarburization by blowing oxygen gas, and burner heating.

Al heating was carried out in a degree of vacuum of 250 Torr for 4minutes after one minute had passed from the start of discharging gas toattain the vacuum condition, while the distance G from the lance to themolten steel surface was set at 3.5 m and the flow rate of oxygen gaswas set at 3300 Nm³ /Hr. In this operation, the cavity depth L was 205mm, the distance N from the lower end of the dipping portion to themolten steel surface in the vacuum vessel was 1400 mm, and the distance(dipping depth) H from the lower end of the dipping portion to themolten steel surface outside the vacuum vessel was 450 mm. The flow rateof Ar of bottom-blow was 500 Nl/min, and Al was charged every one minutein the gas blowing and heating treatments for 4 minutes. The amount ofAl charged in this process was 450 kg in total. As a result, thetemperature of molten steel was raised by 40° C. at the heat transferefficiency of 98.2%.

After that, the distance H was set at 230 mm, and the flow rate of Arwas increased to 750 Nl/min. Then, the molten steel was stirred for 1.5min, so that slag of Al₂ O₃ in the vacuum vessel was made to flowoutside the vessel completely.

Successively, the degree of vacuum was set at 170 Torr, and oxygen gaswas blown into the molten steel for the purpose of decarburization for 4minutes. In this case, the distance G was set at 3500 mm, and the flowrate of oxygen gas was 3300 Nm³ /Hr. In the above treatment, the cavitydepth L was 205 mm, the distance N was 1.5 m, and the distance H(dipping depth) was 450 mm. While the flow rate of Ar of bottom-blow wasset at 700 Nl/min, the carbon concentration was lowered to a value from725 to 415 ppm. In this case, the decarburizing oxygen efficiency was91%.

After the above processing had been completed, the vacuum was maintainedat 200 Torr, and alloy was added to molten steel for the adjustment ofcomposition while burner heating was being conducted. In this case,burner heating was conducted for 5 minutes under the followingcondition. The distance G was set at 4500 mm, the flow rate of LPG was120 Nm³ /Hr, and the flow rate of oxygen gas was 120 Nm³ /Hr. As aresult, the temperature of molten steel was decreased only by 2° C.during the adjustment of composition.

Example 9

Using a straight barrel type vacuum refining apparatus, thespecification of which was the same as that of Example 5, ultra lowcarbon steel was processed in the following manner. Molten steel in thevacuum vessel of the above apparatus, the carbon content of which wasadjusted to 415 ppm in the process of refining in a converter, wassubjected to Al heating and burner heating.

Al heating was carried out in a degree of vacuum of 250 Torr for 4minutes after one minute had passed from the start of discharging gas toattain the vacuum condition, while the distance G from the lance to themolten steel surface was set at 3,500 mm and the flow rate of oxygen gaswas set at 3,300 Nm³ /Hr. In this treatment, the cavity depth L was 205mm, the distance N from the lower end of the dipping portion to themolten steel surface in the vacuum vessel was 1,400 mm, and the distance(dipping depth) H from the lower end of the dipping portion to themolten steel surface outside the vacuum vessel was 450 mm. The flow rateof Ar of bottom-blow was 500 Nl/min, and Al was charged into moltensteel every one minute in the heating process for 4 minutes. The amountof Al charged in this treatment was 450 kg in total. As a result, thetemperature of molten steel was raised by 40° C. at the heat transferefficiency of 98.2%.

After that, the distance H was set at 230 mm, and the flow rate of Arwas increased to 750 Nl/min. Then, the molten steel was stirred for 1.5min, so that slag of Al₂ O₃ in the vacuum vessel was made to flowoutside the vessel completely.

After the temperature had been raised by the above treatment, the degreeof vacuum was maintained at 200 Torr, and alloy was added to moltensteel for the adjustment of composition while burner heating was beingconducted. In this case, burner heating was conducted for 5 minutesunder the following condition. The distance G was set at 4500 mm, theflow rate of LPG was 120 Nm³ /Hr, and the flow rate of oxygen gas was120 Nm³ /Hr. As a result, the temperature of molten steel was decreasedonly by 2° C. during the adjustment of composition.

POSSIBILITY OF INDUSTRIAL USE

According to the present invention, at the beginning of processing inwhich the carbon concentration is high, it is possible to feed oxygenwhile the decarburizing efficiency is high and there is no adheringmetal. Accordingly, it becomes possible to conduct refining fordecarburization effectively so that the carbon concentration can belowered to a value in an ultra low carbon region. Also, it becomespossible to conduct Al heating at a high thermal efficiency. Further,when a desulfurizing refining agent is fed from a lance to molten steeltogether with carrier gas, it is possible to conduct an effectivedesulfurization refining. Accordingly, it is possible to provide ahighly beneficial effect by the molten steel refining method of thepresent invention.

We claim:
 1. A method of refining molten steel tapped from a converter,by a straight barrel type vacuum refining apparatus, comprising thesteps of:charging molten steel tapped from a converter, the carboncontent of which is not more than 0.1 weight %, into a ladle of thestraight barrel type vacuum refining apparatus; dipping an open lowerend portion of a vacuum vessel of the refining apparatus into moltensteel in the ladle to a predetermined depth so as to form a dippingportion of the vacuum vessel; maintaining a degree of vacuum of 105 to195 Torr in a space in the vacuum vessel; blowing gas for stirringmolten steel from a bottom of the ladle; and blowing oxygen gas fordecarburization to molten steel from a top-blowing lance capable offreely moving upward and downward inserted into the vacuum vessel via aninsertion hole formed on a ceiling of the vacuum vessel so that acavity, the depth of which is 150 to 400 mm, can be formed on a surfaceof the molten steel in the vacuum vessel.
 2. A method of refining moltensteel according to claim 1, wherein a distance from the lower end of thedipping portion of the vacuum vessel to the surface of molten steel inthe vacuum vessel is maintained in a range from 1.2 to 2 m.
 3. A methodof refining molten steel according to claim 1, wherein the dippingportion of the vacuum vessel is raised by a distance of 0.2 H to 0.6 Hafter the treatment of decarburization conducted by blowing oxygen, withrespect to the distance H from the lower end of the vacuum vessel in theprocess of blowing oxygen for decarburization, to the surface of moltensteel outside the vacuum vessel.
 4. A method of refining molten steeltapped from a converter, by a straight barrel type vacuum refiningapparatus, comprising the steps of:charging molten steel tapped from aconverter into a ladle of the straight barrel type vacuum refiningapparatus; dipping an open lower end portion of a vacuum vessel of therefining apparatus into the molten steel in the ladle by a predetermineddepth so as to form a dipping portion of the vacuum vessel; maintaininga vacuum of 100 to 300 Torr in a space in the vacuum vessel; blowing gasfor stirring molten steel from a bottom of the ladle; charging Al alloyinto the vacuum vessel; and blowing oxygen gas from a top-blowing lancecapable of freely moving upward and downward inserted into the vacuumvessel via an insertion hole formed on a ceiling of the vacuum vessel sothat the Al alloy melted in the molten steel can be burned to heat themolten steel.
 5. A method of refining molten steel according to claim 4,wherein a cavity, the depth of which is 50 to 400 mm, is formed on thesurface of molten steel in the vacuum vessel.
 6. A method of refiningmolten steel according to claim 4, wherein a distance from the lower endof the dipping portion of the vacuum vessel to the surface of moltensteel in the vacuum vessel is maintained in a range from 1.2 to 2 m. 7.A method of refining molten steel according to claim 4, wherein thedipping portion of the vacuum vessel is raised by a distance of 0.2 H to0.6 H after a burning period of Al alloy, with respect to the distance Hfrom the lower end of the vacuum vessel in the burning period of Alalloy, to the surface of molten steel outside the vacuum vessel.
 8. Amethod of refining molten steel tapped from a converter, by a straightbarrel type vacuum refining apparatus, comprising the steps of:chargingmolten steel tapped from a converter into a ladle of the straight barreltype vacuum refining apparatus; dipping an open lower end portion of avacuum vessel of the refining apparatus into the molten steel in theladle to a predetermined depth so as to form a dipping portion of thevacuum vessel; maintaining a vacuum of 120 to 400 Torr in a space in thevacuum vessel; and blowing a desulfurizing agent to the molten steel inthe vacuum vessel together with carrier gas from a top-blowing lancecapable of freely moving upward and downward inserted into the vacuumvessel via an insertion hole formed on a ceiling of the vacuum vessel,and also blowing gas for agitation into the molten steel from a lowerportion of the ladle so that the molten steel can be desulfurized.
 9. Amethod of refining molten steel according to claim 8, wherein a moltensteel surface arrival speed of carrier gas to blow the desulfurizingagent is in a range from 10 m/sec to Mach
 1. 10. A method of refiningmolten steel according to claim 8, wherein a distance from the lower endof the dipping portion of the vacuum vessel to the surface of moltensteel in the vacuum vessel is maintained in a range from 1.2 to 2 m. 11.A method of refining molten steel tapped from a converter, by a straightbarrel type vacuum refining apparatus, comprising the steps of:chargingmolten steel tapped from a converter into a ladle of the straight barreltype vacuum refining apparatus; dipping an open lower end portion of avacuum vessel of the refining apparatus into the molten steel in theladle by a predetermined depth so as to form a dipping portion of thevacuum vessel; maintaining a vacuum of 100 to 400 Torr in a space in thevacuum vessel; and blowing oxygen gas and combustion improving gas ofhydrocarbon onto the surface of molten steel in the vacuum vessel from atop-blowing lance capable of freely moving upward and downward insertedinto the vacuum vessel via an insertion hole formed on a ceiling of thevacuum vessel.
 12. A method of refining molten steel according to claim11, wherein a distance from the end of the top-blowing lance to thesurface of molten steel in the vacuum vessel is 3.5 to 9.5 m.
 13. Amethod of refining molten steel tapped from a converter, by a straightbarrel type vacuum refining apparatus, comprising the steps of:chargingmolten steel tapped from a converter, the carbon content of which is notmore than 0.1 weight %, into a ladle of the straight barrel type vacuumrefining apparatus; dipping an open lower end portion of a vacuum vesselof the refining apparatus into the molten steel in the ladle to apredetermined depth so as to form a dipping portion of the vacuumvessel; maintaining a degree of vacuum of 100 to 300 Torr in a space inthe vacuum vessel; blowing gas for agitating molten steel from a bottomof the ladle; charging Al alloy into the vacuum vessel; heating themolten steel by burning Al alloy melted in the molten steel when oxygengas is blown into the vacuum vessel from a top-blowing lance capable offreely moving upward and downward inserted into the vacuum vessel via aninsertion hole of the vacuum vessel; blowing oxygen gas fordecarburization of molten steel from the top-blowing lance into thevacuum vessel, the degree of vacuum of which is maintained at 105 to 195Torr, while a cavity, the depth of which is 150 to 400 mm, is formed byblowing oxygen gas on the surface of heated molten steel in the degreeof vacuum vessel; and maintaining a space in the vacuum vessel in a highvacuum condition, the degree of vacuum of which is not more than 100Torr, so as to conduct degassing treatment on the molten steel that hasbeen subjected to decarburization treatment.
 14. A method of refiningmolten steel according to claim 13, wherein a cavity, the depth of whichis 50 to 400 mm, is formed on the surface of molten steel in the vacuumvessel when oxygen gas is blown from the top-blowing lance into thevacuum vessel so as to heat the molten steel by burning Al alloy meltedin the molten steel.
 15. A method of refining molten steel according toclaim 13, wherein the dipping portion of the vacuum vessel is raised bya distance of 0.2 H to 0.6 H before blowing oxygen gas into molten steelfor conducting decarburization treatment, with respect to a distance Hfrom the lower end of the dipping portion of the vacuum vessel in aperiod of burning Al alloy to the surface of molten steel outside thevacuum vessel.
 16. A method of refining molten steel according to claim13, wherein a distance from the lower end of the dipping portion of thevacuum vessel to the surface of molten steel in the vacuum vessel ismaintained in a range from 1.2 to 2 m when molten steel is heated byburning Al alloy and subjected to decarburization by blowing oxygen gas.17. A method of refining molten steel tapped from a converter, by astraight barrel type vacuum refining apparatus, comprising the stepsof:charging molten steel tapped from a converter, into a ladle of thestraight barrel type vacuum refining apparatus; dipping an open lowerend portion of a vacuum vessel of the refining apparatus into the moltensteel in the ladle by a predetermined depth so as to form a dippingportion of the vacuum vessel; maintaining a degree of vacuum of 100 to300 Torr in a space in the vacuum vessel; blowing gas for stirringmolten steel from a bottom of the ladle; raising the temperature ofmolten steel by burning Al alloy melted in the molten steel when Alalloy is charged into the vacuum vessel and oxygen gas is blown from atop-blowing lance capable of moving upward and downward inserted intothe vacuum vessel via an insertion hole formed on a ceiling of thevacuum vessel; conducting hydrogen removal treatment on the moltensteel, the temperature of which is raised while the space in the vacuumvessel is maintained in a high vacuum condition of not more than 100Torr; and conducting desulfurization processing on the molten steel whenthe space in the vacuum vessel is maintained in a degree of vacuum of120 to 400 Torr and a desulfurizing agent is blown from the top-blowinglance to the molten steel in the vacuum vessel together with carrier gasand also when a gas for stirring molten steel is blown into molten steelfrom a bottom of the ladle.
 18. A method of refining molten steelaccording to claim 17, wherein a distance from the lower end of thedipping portion of the vacuum vessel to the surface of molten steel inthe vacuum vessel is maintained in a range from 1.2 to 2 m.
 19. A methodof refining molten steel according to claim 17, wherein the dippingportion of the vacuum vessel is raised by a distance of 0.2 H to 0.6 Hbefore the degassing treatment conducted in a high vacuum condition,with respect to a distance H from the lower end of the dipping portionof the vacuum vessel in a period of burning Al alloy to the surface ofmolten steel outside the vacuum vessel.
 20. A method of refining moltensteel according to claim 17, wherein a cavity, the depth of which is 50to 400 mm, is formed on the surface of molten steel in the vacuum vesselwhen oxygen gas is blown from the top-blowing lance into the vacuumvessel so as to heat the molten steel by burning Al alloy melted in themolten steel.
 21. A method of refining molten steel tapped from aconverter, by a straight barrel type vacuum refining apparatus,comprising the steps of:charging molten steel tapped from a converter,the carbon content of which is not more than 0.1 weight %, into a ladleof the straight barrel type vacuum refining apparatus; dipping an openlower end portion of a vacuum vessel of the refining apparatus into themolten steel in the ladle by a predetermined depth so as to form adipping portion of the vacuum vessel; maintaining a degree of vacuum of100 to 300 Torr in a space in the vacuum vessel; blowing gas forstirring molten steel from a bottom of the ladle; charging Al alloy intothe vacuum vessel; blowing oxygen gas from a top-blowing lance capableof freely moving upward and downward inserted into the vacuum vessel viaan insertion hole formed on a ceiling of the vacuum vessel so that Alalloy melted in the molten steel can be burned to heat the molten steel;blowing oxygen gas for decarburization from the top-blowing lance ontomolten steel in the vacuum vessel when the space in the vacuum vessel ismaintained in a vacuum condition of 105 to 195 Torr and a cavity, thedepth of which is 150 to 400 mm, is formed by blowing oxygen gas on thesurface of the heated molten steel in the vacuum vessel; conductingdegassing treatment on the molten steel, which has been subjected todecarburization treatment, while the space in the vacuum vessel ismaintained in a high vacuum condition of not more than 100 Torr;conducting desulfurization processing on the molten steel when the spacein the vacuum vessel is maintained in a degree of vacuum of 120 to 400Torr and a desulfurizing agent is blown from the top-blowing lance tothe molten steel in the vacuum vessel together with carrier gas; andblowing both oxygen gas and combustion improving gas of hydrocarbon fromthe top-blowing lance to the surface of the desulfurized molten steel inthe vacuum vessel so as to heat it while the space in the vacuum vesselis maintained in a degree of vacuum of 100 to 400 Torr.
 22. A method ofrefining molten steel according to claim 21, wherein the distance fromthe lower end of the dipping portion of the vacuum vessel to the surfaceof molten steel in the vacuum vessel is maintained in a range from 1.2to 2 m in the heating treatment of molten steel conducted by burning Alalloy, the decarburizing treatment conducted by blowing oxygen gas, orthe desulfurizing treatment.
 23. A method of refining molten steelaccording to claim 21, wherein a distance from the end of thetop-blowing lance to the surface of molten steel in the vacuum vessel ismaintained in a range from 3.5 to 9.5 m when molten steel is heated byburning oxygen gas and a combustion improving gas of hydrocarbon.
 24. Amethod of refining molten steel according to claim 21, wherein thedipping portion of the vacuum vessel is raised by a distance from 0.2 Hto 0.6 H before the molten steel is subjected to decarburization byblowing oxygen gas, with respect to a distance H from the lower end ofthe dipping portion of the vacuum vessel in a period of burning Al alloyto the surface of molten steel outside the vacuum vessel.
 25. A methodof refining molten steel according to claim 21, wherein a cavity, thedepth of which is 50 to 400 mm, is formed on the surface of molten steelin the vacuum vessel when oxygen gas is blown into the vacuum vesselfrom the top-blowing lance so as to burn Al alloy melted in the moltensteel to heat the molten steel.